The invention relates generally to gas turbine engines and, more particularly, to an impingement manifold for supplying cooling air to the exterior surface of a casing of a gas turbine engine.
It is understood in the gas turbine art that engine efficiency is improved by minimizing the leakage of hot gases past the turbine. Leakage air does not contribute to the power extracted by the turbine and consequently represents a loss of overall efficiency. Therefore, much effort has been given to limiting clearance between rotor and stator components.
Typically, the prior art has supplied cooling air to gas turbine components to control thermal growth of the turbine casing to minimize the operating clearances. Cooling air is supplied to circular spray bars which impinge cooling air upon circular components surrounding a row of turbine blades. For example U.S. Pat. No. 4,214,851 issued July 29, 1980, to Tuley, et al., and assigned to the assignee of the present invention, discloses a cooling air manifold of an annular shape with radial holes to supply cooling air to stator vanes. To further control cooling, the air supply has been controlled as described in, for example, U.S. Pat. No. 4,230,436, issued to Davison and assigned to the assignee of the present invention. In Davison, two sources of air are mixed according to the demand to provide a cooling flow in response to measured engine operating parameters.
As cooling air passes through the cooling manifold it picks up heat from the turbine casing. The amount of heat absorbed increases as the length of travel of the air in close proximity to the casing increases. One significant limitation of the prior art manifold construction is that a temperature gradient was established circumferentially around the annular spray bars due to the heating of the cooling air as it flows circumferentially from the supply tube around the engine. Uneven cooling allows uneven thermal growth, which limits the amount of performance improvement which can be realized, because the engine design is required to include greater clearance to prevent rubbing of rotor blade tips against the stator structure.